The Fischer-Tropsch reaction involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to higher aliphatic alcohols. The methanation reaction was first described by Sabatier and Senderens in 1902. The later work of Fischer and Tropsch dealing with higher hydrocarbon synthesis was described in Brennstoff-Chem, 7, 97 (1926).
The reaction is highly exothermic and care must be taken to design reactors for adequate heat exchange capacity as well as the ability to continuously produce and remove the desired range of hydrocarbon products. The process has been considered for the conversion of carbonaceous feedstocks, e.g., coal or natural gas, to higher value liquid fuel or petrochemicals. The first major commercial use of the Fischer-Tropsch process was in Germany during the 1930's. More than 10,000 B/D of products were manufactured with a cobalt based catalyst in a fixed-bed reactor. This work has been described by Fischer and Pichler in German Patent No. 731,295 issued Aug. 2, 1936.
Commercial practice of the Fischer-Tropsch process has continued in South Africa in the SASOL plants. These plants use iron based catalysts and produce gasoline in fluid-bed reactors and wax in fixed-bed reactors.
Research aimed at the development of more efficient CO hydrogenation catalysts and reactor systems is continuing. In particular, a number of studies describe the behavior of iron, cobalt or ruthenium based catalysts in slurry reactors together with the development of catalyst compositions and improved pretreatment methods specifically tailored for that mode of operation.
Farley et al in The Institute of Petroleum, vol. 50, No. 482, pp. 27-46, February (1984) describe the design and operation of a pilot-scale slurry reactor for hydrocarbon synthesis. Their catalysts consisted of precipitated iron oxide incorporating small amounts of potassium and copper oxides as promoters. These catalysts underwent both chemical and physical changes during activation with synthesis gas in the slurry reactor.
Slegeir et al in Prepr. ACS Div. Fuel Chem, vol. 27, p. 157-163 (1982) describe the use of supported cobalt catalysts for the production of hydrocarbons from synthesis gas at pressures above 500 psi in a continuous stirred tank (CSTR) slurry reactor.
Brennan et al in U.S. Pat. No. 4,605,678 issued on Aug. 12, 1986 describe a process for removing catalyst fines from the wax product produced in a slurry Fischer-Tropsch reactor. Their process comprises removing the wax product from the reactor and separating the catalyst fines by passing the wax through a high gradient magnetic field, whereby the catalyst fines are held by a magnetized filter element and the wax product passes through unhindered to form a purified wax product. The separated catalyst fines are returned to the reactor by backwashing the filter element.
Rice et al in U.S. Pat. No. 4,659,681 issued on Apr. 21, 1987 describe the laser synthesis of iron based catalyst particles in the 1-100 micron particle size range for use in a slurry Fischer-Tropsch reactor.
Dyer et al in U.S. Pat. No. 4,619,910 issued on Oct. 28, 1986 and U.S. Pat. No. 4,670,472 issued on June 2, 1987 and U.S. Pat. No. 4,681,867 issued on July 21, 1987 describe a series of catalysts for use in a slurry Fischer-Tropsch process in which synthesis gas is selectively converted to higher hydrocarbons of relatively narrow carbon number range. Reactions of the catalyst with air and water and calcination are specifically avoided in the catalyst preparation procedure. Their catalysts are activated in a fixed-bed reactor by reaction with CO+H.sub.2 prior to slurrying in the oil phase in the absence of air.
Fujimoto et al in Bull. Chem. Soc. Japan, vol. 60, pp. 2237-2243 (1987) discuss the behavior of supported ruthenium catalysts in slurry Fischer-Tropsch synthesis. They indicate that the catalyst precursors were ground to fine powders (&lt;150 mesh), calcined if needed, and then activated in flowing hydrogen before addition to a degassed solvent and subsequent introduction to the slurry reactor.
The organic product for the slurry Fischer-Tropsch process contains olefins, paraffins and oxygenated hydrocarbons with carbon numbers from 1 to well over 100. Only those compounds with high vapor pressure at reaction conditions will readily be removed with the effluent gas stream. The relatively non-volatile molecules such as C20+ paraffin wax will remain in the slurry oil phase. During continuous operation it is necessary to remove these non-volatile products in a continuous manner in order to prevent excessive build-up in the reaction zone. This is especially important if the process is being conducted for the selective production of these high molecular weight (non-volatile) hydrocarbons.
Farley et al (vida supra) conducted numerous laboratory tests to determine the best method by which to withdraw Fischer-Tropsch wax from a slurry reactor, which was both capable of high withdrawal rates and yet would efficiently retain the catalyst within the reaction system for subsequent use. Magnetic separation techniques, sintered metal and woven metal filters were shown to be unacceptable for use at the severe temperatures and pressures used. These systems gave limited filter flux rates (quantity of wax filtered per unit of filter surface area per unit of time) that was probably due to partial plugging of the filter by the powdered iron based catalysts that were being used.
An object of this invention is the preparation of a powdered catalyst which is free of sub 1 micron particles, for use in a continuous Fischer-Tropsch process for the production and continuous withdrawal of hydrocarbon wax. The use of the pretreatment procedure of the instant invention precludes the need for the complex and costly magnetic separation schemes that have been disclosed and allows the use of relatively inexpensive filtration techniques that have heretofore been shown to be of difficult and therefore of marginal utility.
This invention is applicable to catalyst or carrier wherein sub micron particles adhere to the catalyst or carrier particles as a result of the carrier or catalyst preparation. Thus, inorganic oxide carriers of appropriate size may be prepared by crushing or grinding techniques well known to the art. Catalytic metals are then incorporated onto the particles. Alternatively, catalysts may be prepared by incorporating catalytic metals onto particulate carriers of a relatively larger size and then the catalyst is crushed or ground to form the powdered catalyst of appropriate size. During the crushing or grinding steps the mechanical sizing step, very fine submicron particles are formed that tend to adhere to the catalyst and carrier particles.